I joined the Atomistic Simulation Group as a Research Associate in July 2013. My main research interest is structural defects and mechanical properties using atomistic simulations. I have recently been working on the behaviour of fission gases and irradiation defects in uranium dioxide, as well as radiation damage in fluorapatite.

Before coming to London I did a PhD on the mechanical behaviour of UO₂ at the atomistic length-scale, with the École Centrale Paris and the Thermodynamic and Thermochemistry Modelling Lab at the CEA centre at Saclay. I studied some deformation mechanisms in UO₂ at the atomic length-scale. One of them was the structural phase transitions that occur in high stress conditions. Depending on the stress state, the initial Fluorite structure can change to different structures. For example a <100> uniaxial tension causes a transition to the Scrutinyite structure, whereas a <110> tension induces a transition to the Rutile structure at about the same stress of 7 GPa.

Such high stress is found during fracture processes. Indeed both phase transitions occur at the crack tip during loading, depending on the loading axis. Both structures are related to very different fracture behaviour. Propagation in a {100} planes is caused by sub-crack opening in Scrutinyite secondary phases and then coalescing. On the other hand, when a crack starts propagating in a {110} plane, it deviates and propagates by simple decohesion in the secondary Rutile phase.

I also investigated dislocations glide, and the related critical stresses as a function of temperature for edge <110> dislocations. At lower temperatures, the easiest glide system is <110>{100} followed by <110>{111} and then <110>{110} having a much higher critical stress. This changes at about 1500 K, where the critical stresses for {100} and {111} planes are very close, which corresponds to the experimentally reported fragile to ductile transition temperature.

Before that I did a Masters in computational physics at the Université Montpellier II (France). My thesis was on the effect of helium bubbles on the structural properties of a CJ1 glass matrix. CJ1 is a simplified borosilicate glass, used as a surrogate for the more complex R7T7 glass used by the French nuclear industry as a radioactive waste containment matrix.

Publications

• P.C.M. Fossati, L. Van Brutzel, A. Chartier and J-P. Crocombette, “Simulation of uranium dioxide polymorphs and their phase transitions”, Physical Review B, 88 (2013) 214112. doi:10.1103/PhysRevB.88.214112.
• P.C.M. Fossati, L. Van Brutzel and B. Devincre “Molecular dynamics simulation of dislocations in uranium dioxide”, Journal of Nuclear Materials, 443 (2013) 359–365. doi:10.1016/j.jnucmat.2013.07.059

PhD Thesis

• Contribution à l’étude des propriétés mécaniques du combustible nucléaire: modélisation atomistique de la déformation du dioxyde d’Uranium: Contribution to the study of mechanical properties of nuclear fuel–atomistic modeling of the deformation of uranium dioxide.